The present invention relates to a color cathode ray tube, particularly, to a color cathode ray tube having a panel with improved flatness of the effective region, having a vacuum envelope exhibiting a mechanical strength high enough to fully withstand the atmospheric pressure and to fully maintain a curved surface of the shadow mask, and exhibiting a satisfactory brightness at the peripheral portion of the panel.
In general, a color cathode ray tube comprises a vacuum envelope 4 consisting of a glass panel 3 and a funnel, as shown in FIG. 1. In the panel 3, a skirt portion 2 is formed at the peripheral portion of a face plate 1 having a curved surface and a substantially rectangular effective portion. The glass funnel is bonded to the skirt portion 2 of the panel 3 so as to constitute the vacuum envelope 4. A phosphor screen 5 consisting of a black material layer which does not emit light and three-color phosphor layers is formed on the inner surface of an effective portion 1 of the face plate. A shadow mask 9 is arranged inside the panel 3 in a manner to face the phosphor screen 5. The shadow mask 9 consists of a mask body 7 having a substantially rectangular effective face 6 and having a large number of electron beam-passing holes formed therein and a mask frame 8 arranged in a peripheral portion of the mask body 7. On the other hand, an electron gun assembly 11 is arranged within a neck 10 of the funnel. Three electron beams 12B, 12G, 12R emitted from the electron gun assembly 11 are deflected by the magnetic field generated from a deflecting device 13 mounted outside the funnel so as to scan the phosphor screen 5 in both horizontal and vertical directions via the shadow mask 9, with the result that a color picture image is displayed on the rectangular effective face 6.
For displaying a color picture image free from color deviation on the phosphor screen 5 in the color cathode ray tube described above, the electron beams 12B, 12G, 12R passing through the electron beam-passing holes formed in the mask body 7 of the shadow mask 9 are required to perform landing accurately on the three-color phosphor layers of the phosphor screen 5. To meet this requirement, it is necessary to maintain accurately the positional relationship between the panel 3 and the shadow mask 9.
In order to improve the visibility of the color cathode ray tube, the outer surface in the effective area of the panel is required in recent years to be flattened to have a very small curvature. The curvature in the inner surface of the effective area is also required to be diminished in view of the moldability of the panel and the visibility of the color cathode ray tube.
However, in a color cathode ray tube having a flattened panel as described above, a serious problem to be considered is whether the vacuum envelope including the particular panel has a mechanical strength high enough to withstand the atmospheric pressure. If the thickness of the panel is increased in an attempt to ensure a mechanical strength high enough to withstand the atmospheric pressure, the transmittance of the effective portion is lowered, leading to deterioration of brightness.
Further, for permitting the electron beams to land accurately on the three-color phosphor layers of the phosphor screen 5 mounted on the inner surface of the effective portion of the panel, the effective surface of the mask body having the electron beam-passing holes formed therein is required to have a curvature diminished appropriately to conform with the inner surface in the effective portion of the panel. However, if the curvature in the effective surface of the mask body is diminished, the mechanical strength serving to keep the mask curvature unchanged is deteriorated, leading to deformation of the shadow mask. As a result, the color purity is likely to be deteriorated.
It should also be noted that, because of the operating principle of the color cathode ray tube of shadow mask type, the electron beam reaching the phosphor screen through the electron beam-passing holes of the shadow mask is at most 1/3 of all the electron beams emitted from the electron gun. The remaining electron beams, which do not pass through the beam-passing holes of the shadow mask, impinge on the shadow mask so as to be converted into a heat energy and, thus, to heat the shadow mask. When heated, the shadow mask is thermally expanded so as to bring about a doming problem that the effective area of the shadow mask is swollen toward the phosphor screen. As a result, the distance between the inner surface in the effective portion of the panel and the effective area of the mask body is changed. If the change in the distance exceeds an allowable range, the electron beam fails to land accurately on the phosphor layer, leading to deterioration of the color purity. The magnitude of the mis-landing caused by the thermal expansion of the shadow mask depends on the brightness of the image pattern, the continuing time of the image pattern, etc. Particularly, in the case of locally displaying an image pattern of a high brightness, a local doming problem takes place, giving rise to a large local mis-landing in a short time.
The mis-landing caused by the local doming problem is increased where the curvature is diminished in the effective area of the mask body. It follows that it is unavoidable to cope with the deterioration of the color purity caused by the local doming problem in order to flatten the effective area of the panel. It should also be noted that, if the outer surface alone in the effective portion of the panel is flattened, it is unavoidable for a difference in thickness between central portion and the peripheral portion of the panel to be increased, giving rise to a large difference in the light transmittance between the central portion and the peripheral portion of the panel. Naturally, a difference in brightness between the central portion and the peripheral portion of the panel is also increased, giving rise to deterioration in visibility of the cathode ray tube.
As described above, if the curvature of the outer surface in the effective portion of the panel is much diminished to make the outer surface close to a flat surface in order to improve the visibility of the cathode ray tube, a serious problem is raised in terms of the mechanical strength of the vacuum envelope because the vacuum envelope is required to withstand the atmospheric pressure. On the other hand, if the thickness of the panel is increased in an attempt to allow the vacuum envelope to withstand the atmospheric pressure, the transmittance in the effective portion is lowered, leading to deterioration of the brightness.
Further, if the curvature in the effective portion of the mask body is diminished to conform with the flattening in the effective portion of the panel, the mechanical strength serving to hold the curved surface of the mask body is lowered, giving rise to various problems such as deformation of the shadow mask, and deterioration of the color purity resulting from mis-landing of the electron beam caused by a local doming problem.